1. Field of the Invention
The present invention relates to a drawing system in which a thermoplastic resin such as polypropylene, polyethylene or the like is molded into a shape of a film through a die by melt-extrusion with an extruder, a not-oriented film thus molded is drawn at a temperature equal to or lower than a melting point in longitudinal and lateral directions to orient molecules, and to an oriented film producing facility which comprises the drawing system, a measuring apparatus for measuring a thickness and an degree of orientation of a drawn film, and in which facility a predetermined control operation is performed on the drawing system based on measured values.
The present invention further relates to a high speed measuring method for measuring a birefringence, especially a birefringence, whose retardation which is larger than a measuring wavelength.
2. Description of the Background Art
A film used for packaging and the like has conventionally been produced in such a manner that a not-oriented film which is already molded into a shape of a thin film through die mounted on an extruder is subjected to drawing in longitudinal and lateral directions to orient molecules, whereby a bi-axially oriented film, whose mechanical characteristic, especially a strength is improved, made of polypropylene, vinylidene chloride resin, vinyl chloride resin, polystyrene, polyethylene terephthalate, polyethylene naphthalate, polyethylene, polyamide or the like is produced.
Here, the not-oriented film to be bi-axially oriented and the like are categorized in a crystalline resin and an amorphous resin and in the case of the crystalline resin a film is extruded at a temperature equal to or higher than a melting point and subjected to a rapid cooling while an amorphous condition is maintained in order to facilitate orientation. Then, the not-oriented film is drawn at a temperature equal to or lower than a melting point for orientation.
In the case of the amorphous resin, a film is extruded by an extruder at a temperature higher than a glass transition temperature so as to give the maximum degree of transparency. In order to orient the not-oriented film, the not-oriented film is cooled to a temperature in an elasticity range and drawn at the temperature for orientation.
A drawing machine which is used for orientation of a not-oriented film is, in a broad sense, categorized in a sequential bi-axial drawing type in which a longitudinal drawing is followed by a lateral drawing and a simultaneous bi-axial drawing type in which longitudinal and lateral drawings are simultaneously conducted. These two type machines are desired that a film which is oriented in a uniform manner in the two directions can be produced. In the case of the sequential drawing, while drawings are conducted in two stages, it is desirable that draw ratios of longitudinal and lateral directions and a temperature in respective stages can independently be changed in order to improve controllability on an degree of orientation and that the draw ratios can independently be changed in simultaneous drawing as well.
Recently, quality improvement on a film has increasingly been desired and it has furthermore been demanded that a film can be subjected to uniform drawing throughout the entire surface in both way, lengthwise and widthwise.
In order to cope with such desire and demand, there have been used a measuring apparatus for measuring a film thickness included in a drawing system and a film automatic control apparatus by which an extruder and a die are subjected to a predetermined control while a thickness is measured by the measuring apparatus.
The film automatic thickness control apparatus, as shown in FIG. 13, comprises: an extruder 50 which enables adjustment of a rotation number of a screw thereof, a die 51 mounted to the extruder, a die adjusting apparatus 52 for adjusting an opening degree of an extrusion mouth, a base film shaping apparatus 53 in which a not-oriented film is cooled to be hard, drawing machines 54, 55 which draws the not-oriented film which has been cooled to be hard is drawn in longitudinal and lateral directions, and a take-up apparatus 56 which takes up a drawn film, and .beta. ray thickness gauges 57, 57 for measuring a thickness are respectively deployed at positions upstream of the longitudinal drawing machine 54 and downstream of a lateral drawing machine 55.
A thickness of a film is measured by these gauges 57, 57 and not only is a screw rotational number of the extruder 50 controlled based on a measuring data but an opening degree of the die 51 is controlled by an adjust bolt or the like.
There have been known measuring methods for birefringence as one of the means for evaluating a degree of orientation in an oriented high polymer film. Among such methods, if a retardation caused by a birefringence is larger than a measuring wavelength, a birefringence has to be computed from a spectrum of transmitted light when a high polymer film which is a specimen showing a birefringence is placed between a polarizing elements and therefore, as a high speed measuring method for measuring a birefringence, there has been known a method in which data of transmitted light spectrum measured by a multi-channel spectrometer are analyzed by use of a data processor such as a computer.
In a drawing machine equipped with the above mentioned film automatic thickness control apparatus, not only is a screw rotational speed of an extruder adjusted but an opening degree of a extrusion mouth of a die is adjusted based on measurement of a thickness only.
However, there is difficulty maintaining a thickness and an degree of orientation in a uniform manner along a direction perpendicular to a advancing direction of a base film, that is a width direction of the base film.
If a screw rotational speed is changed, a feed of molten resin is changed and thereby a global thickness of a base film can be controlled.
According to this method, while a thickness and a degree of orientation, as averages, of a drawn film can constantly be maintained, local fluctuations of thickness and degree of orientation cannot be prevented.
If an opening degree of the extrusion mouth of a die is narrowed by an adjust bolt such as a heat bolt, a thickness in a width direction can be controlled, but it entails reduction of a mass per a unit time of a not-oriented film during a time when the die extrusion mouth is narrowed, which is not to maintain a uniform degree of orientation over all the surface area.
In a drawing machine without the film automatic thickness control apparatus, fluctuations of thickness and degree of orientation along not only a width direction, but a longitudinal direction, of a film occurs. Therefore, there is a problem that the film during drawing has a high proportion of break-down by a drawing tension in longitudinal and lateral directions imposed on the film.
If a film is broken, an operation of a drawing system has to be stopped and, a broken film has to be removed, and not only is a time passed but labor is consumed before the system is restarted, which is a problem.
Especially, if a film is broken in a lateral drawing machine, necessary operations cannot be started since a temperature inside the machine is high and thereby there arises a problem that productivity is reduced.
There arises another problem that while a improvement toward a higher speed of drawing is desired for the purpose of cost down, unevenness in thickness and degree of orientation of a film is increased and a probability of film breakdown is increased if drawing is conducted at a high speed.
It has been desired to establish a production method for drawing an oriented film at a speed of 350 m/min or higher, or especially 500 m/min or higher.